It could be exciting near the beginning of the game, if your rival followed you around for a little bit, pointing out things like how to catch a Pokemon, teaching you about double battles, healing your Pokemon, etc. By the time you get to Rustboro (maybe even by Petalburg) it might get a little annoying though, so maybe it's for the best that they didn't go with this route and have Brendan/May follow you around the majority of the game.